Fluoropolymers are widely used to protect metallic surfaces from corrosion due to weathering or to chemical agents. Metal industrial equipment can be protected from the corrosion using coatings based on fluoropolymers. Such coating guarantees chemical inertness towards aggressive environments and may need to be applied directly on various surfaces such as stainless and carbon steel, aluminium, copper, bronze, brass and special alloys. The coating must also be mechanically resistant to withstand friction and impacts during operations and must not crack or wear excessively during normal conditions. Fluorinated materials, such as those based on ethylene/chlorotrifluoroethylene copolymers (ECTFE), are available to be used for coating, particularly those having a Melt Flow Index (MFI) above 10 g/min.
Coatings can be applied on the surfaces to protect via various coating processing methods, including employing polymer dispersions in organic or aqueous solvents and by electrostatic powder coating. However, defects which may occur in a layer of coating, such as pinholes, cracks and porosity, diminish its chemical and mechanical resistance. To reduce the possibility of damages and to minimize the potential weaknesses, multiple layers of polymer coatings are usually applied on the surface of the metal equipment until the desired thickness is obtained. This requires lengthy operation, as an overcoating layer can only be applied after the lower layer has dried. Moreover, adhesion between multiple layers can be imperfect, leading to blistering, delamination and buckling phenomena that ultimately determine poor performance of the protective coating.
The main processes for deposition of a fluoropolymer coating are electrostatic powder coating and application of liquid dispersions.
Deposition via electrostatic powder coating requires that the metal surface to be coated is earthed and heated to a temperature above the melting range of the coating polymer. In addition, generally the surface to be coated must be previously degreased and roughened via chemical etching or via mechanical means to improve adherence of the coating (US 2003/0031875 A (AUSIMONT S.P.A.) Feb. 13, 2003.
In alternative, the coating can be applied using a dispersion of the fluoropolymer in an aqueous solvent system. This method does not require earthing of the substrate and is in general simpler, more environmentally friendly and less energy-consuming than electrostatic powder coating. Examples of liquid dispersions suitable for coating are disclosed in US 2009/0018244 A (SOLVAY NORTH AMERICA, LLC) Jan. 15, 2009 Liquid fluoropolymer dispersions can be applied to the surface to be coated using various techniques known to the person skilled in the art, such as dipping, spraying, roll method, doctor blade or flow coat methods. It is advantageous that the liquid compositions be applicable directly to the metal surface, so that surface pre-treatment and application of a primer layer are not needed.
US 2011/213069 A (THREE BOND CO.) Sep. 1, 2011 relates to an aqueous coating composition having as main components component (A), an ethylene-vinyl acetate copolymer resin emulsion, and component (B), two or more PTFE (polytetrafluoroethylene) resin powders having different particle diameters, namely a PTFE resin powder (b-1) having an average particle diameter of 2 to 20 μm and a PTFE resin powder (b-2) having an average particle diameter of 20 to 100 μm.
An objective of the present invention is to provide a liquid composition that can be applied homogeneously to a surface, so as to obtain a fluoropolymer-based protective coating that is uniform and free of imperfections, which might lead to poor protection of the surface from weathering and chemical agents.
Another aim of the present invention is to provide a protective coating for a surface that is easily applicable and does not require the deposition of several layers.